The invention relates to a magnetic field sensor.
A direction-change sensor is known from document EP 545 058 A1, for an accident-recording device that can be used in motor vehicles, with at least two cylindrical coils disposed at right angels to one another and each having a core made of amorphous ferromagnetic metal, with the geometric axes of the coils being aligned essentially horizontally following the installation of the accident data recording device in a motor vehicle, and with the inductances of the coils being controlled by the earth's magnetic field, with essentially similarly constructed oscillators being provided, each comprising one of the coils, and with the amorphometallic cores of the coils being biased in a permanently magnetic fashion to establish the working ranges of the oscillators. The magnetic bias on the coil cores is generated by permanent magnets, preferably in the shape of rods, located In the vicinity of the coils, with the magnetic fields of these permanent magnets penetrating the coils. By the magnetic biasing, frequency changes of the oscillator, caused by the influence of the earth's magnetic field, can be tapped in the linear range of its f/B curve. The determination of the working point of the oscillator by the field of the permanent magnet involves consideration of a practically oriented signal travel signal deviation which results when the earth's magnetic field is superimposed on the bias.
EP 503 370 A1 contains a proposal for the design of the coil for an oscillator controlled by a magnetic field, with a coil body, a winding, and a coil core made of amorphous ferromagnetic metal.
The arrangement according to EP 545 058 A1 has the disadvantage that the desired working point is not stable under automotive operating conditions, directly affecting the quality of the measurement. The measuring system described in the document is subject to temperature drift, because both the magnetic properties of the permanent magnet and the magnetization of the amorphometallic coil core change as a function of temperature. Even when the temperature pattern of the material of which the permanent magnet is made and that of the coil core behave in opposite ways, it is extremely difficult to find pairs of materials in which the influence of temperature on amorphous metal and on the permanent magnet offset one another in the desired working range, so that in practice undesired temperature influences are always superimposed on the measurement result.
In addition, magnetic field sensor systems of this type have the disadvantage of a high calibration cost to set the working point of the oscillator. For calibration, the permanent magnet producing the magnetic bias is moved around in the vicinity of the coil(s) until the position of the permanent magnet that defines the desired working points is determined by measuring the voltage indicated in the coil(s). Then the permanent magnet must be secured in this position. This procedure is too cumbersome for a mass-produced product, and in addition this procedure can be performed only in a magnetically shielded measuring chamber in order to eliminate any superimposition of the earth'magnetic field during the calibration process.
Magnetic field sensor systems with a permanent magnet located in the vicinity of the measuring coil also have the disadvantage that it is difficult to locate the permanent magnet in such a position that its magnetic field passes homogeneously through the coil. For this purpose, special, expensive shaping procedures for the magnet are required. Nonuniform penetration of the coils by the permanent magnet however has a disadvantageous effect on the measurement accuracy that can be achieved with the measuring system.